Gold electroplating process and solution



United States Patent Int. Cl. C23b /28 US. Cl. 204-46 6 Claims This invention relates to an improved gold electroplating process and solution and, more particularly, to such a process and solution which allows a more rapid plating of gold than has been possible heretofore. It is an object of this invention to provide an improved gold electroplating process and solution of such character.

Many commonly utilized gold-plating solutions contain potassium aurocyanide, KAu(Cn) as the gold supply constituent thereof, and are generally operated with a cathode current density ranging from about 3 to amperes per square foot. A current density of about 6 amperes per square foot is generally preferred for most gold electroplating solutions of this type, and results in a typical plating rate of about 0.5 mils per hour.

Another object of this invention is to provide an improved gold electroplating process and solution which allow substantially higher current densities to be employed in the plating process (5-10 times higher than with typical prior art baths), and which accordingly plate gold at a much higher rate.

Still another object of this invention is to provide an improved gold electroplating process and solution that are simple and reliable in operation and economical to utilize.

In accordance with the principles of the present invention, gold from a potassium aurocyanide plating solution is deposited on a substrate at a much higher plating rate than ever obtained heretofore, by using current densities which greatly exceed those found either possible or practical heretofore. Equally significant is the fact that the deposited gold exhibits a smooth, semihard finish, is dull in color in heavy coatings, and is nonporous. The process makes possible the plating of heavy gold deposits (for example, in excess of one mil) while maintaining the desirable metallurgical properties previously discussed.

One example of a preferred formulation for the solution in accordance with the invention is an aqueous solution containing potassium aurocyanide, potassium cyanide and potassium hydroxide in the following proportions:

Grams per liter Potassium aurocyanide, KAu(Cn) 60 Potassium cyanide, KCN 7.5 Potassium hydroxide, KOH 40 This plating solution, when utilized with the preferred method of this invention, is operated at a temperature of about 180 F. with a cathode current density of about 50 amperes per square foot. At this preferred operating temperature and current density, the plating rate of gold from the plating solution is approximately 5 mils per hour, with a cathode eificiency of the order of 98%. The substrate, as is customary, is made the cathode in the electroplating solution, and the anode used must be of an insoluble type, such as stainless steel, carbon, platinum-plated titanium, or platinum-clad tantalum.

As a comparison, one representative prior art, aqueous plating solution has the following composition:

Grams per liter Potassium aurocyanide, KAu(CN) Potassium cyanide, KCN 56 Potassium hydroxide, KOH 5 3,445,352 Patented May 20, 1969 ICC This solution is normally used at a temperature of F. and a cathode current density of 6 amperes per square foot. This type of solution exhibits a useful maximum plating rate of only about 0.5 mils per hour. When an attempt was made to operate this prior art plating solution at a current density above about 10 amperes per square foot, the resulting deposited gold was soft and spongy and, therefore, not of acceptable quality for most applications, particularly not in the plating of electrical components.

The specific example given above for the preferred plating solution has been found to be the most satisfactory. However, it has been determined that satisfactory gold electroplating baths in accordance with the invention can contain the above specified chemical constituents in the following ranges:

Grams per liter Potassium aurocyanide, KAu(CN) 40-80 Potassium cyanide, KCN 6-9 Potassium hydroxide, KOH 32-48 Any plating solution which may be formulated from the ingredients set forth above may be operated in a temperature range from about 160 F. to about F. with cathode current densities in the range of about 40-50 amperes per square foot. Agitation of the solution and/or substrate is preferred, and permits higher cathode current densities and thus faster deposition rates. Various conventional agitation techniques are suitable. All of such plating solutions, whose chemical constituents fall Within the above described ranges and which are operated in accordance with the above described temperature and current density ranges, produce satisfactory semihard, nonporous gold deposits on a suitable substrate at plating rates approximating 5.0 mils per hour. The types of substrate which can be plated are the same as with prior baths of this general type, for example, most conductive materials such as copper or ferrous materials. Also of interest in the semiconductor industry is the gold plating of silicon or germanium wafers which have previously been plated such as with electroless nickel, or the plating of metallized ceramic substrates for semiconductors.

There has been disclosed herein a preferred method of electroplating gold and a suitable plating solution for utilization therewith. The gold electroplating process utilizes current densities far in excess of those known in the prior art and produces semihard, nonporous gold deposits on substrates at plating rates and in deposit thicknesses not obtainable with any known prior art solutions.

While one preferred electroplating process and solution has been described in detail herein, it will be apparent that various modifications of the specific details may be made without departing from the true spirit and scope of the invention.

What is claimed is:

1. A gold-plating solution consisting essentially of about 40 to about 80 grams of potassium aurocyanide, of about 6 to about 9 grams of potassium cyanide and of about 32 to about 48 grams of potassium hydroxide, respectively, per liter of aqueous solution.

2. A gold-plating solution as recited in claim 1, Wherein the concentrations are as follows: about 60 grams of potassium aurocyanide, about 7.5 grams of potassium cyanide, and about 40 grams of potassium hydroxide per liter of aqueous solution.

3. A process for gold plating a substrate, comprising:

immersing the surface of the substrate to be plated in an aqueous solution consisting essentially, in each liter of aqueous solution, of about 40 to about 80 grams of potassium aurocyanide, of about 6 to about 9 grams of potassium cyanide and of about 32 to about 48 grams of potassium hydroxide; and

passing an electric current through the solution with the substrate to be plated being the cathode and the anode being the insoluble type.

4. A process as recited in claim 3, wherein the cathode current density is from about 40 to 50 amperes per square foot.

5. A process as recited in claim 4, wherein the solution is maintained at a temperature from about 160 F. to 180 F.

6. A process for gold plating a substrate, comprising:

immersing the surface of the substrate to be plated in an aqueous solution consisting essentially, in each liter of solution, of about 60 grams potassium auro- 15 cyanide, of about 7.5 grams potassium cyanide, and of about 40 grams potassium hydroxide;

maintaining the temperature of the solution at 180 F.; and

passing an electric current through the solution at a cathode current density of about 50 amperes per square foot with the substrate to be plated being the cathode and the anode being of the insoluble type.

References Cited UNITED STATES PATENTS 2,367,314 1/ 1945 Russell 204-46 XR 2,801,960 8/1957. Seegmiller 204-46 3,156,635 11/1964 Foulke 20446 OTHER REFERENCES Fischer, Johannes et al., Precious Metal Plating, p. 112, 1964.

JOHN H. MACK, Primary Examiner.

G. L. KAPLAN, Assistant Examiner. 

3. A PROCESS FOR GOLD PLATING A SUBSTRATE, COMPRISING: IMMERSING THE SURFACE OF THE SUBSTRATE TO BE PLATED IN AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY, IN EACH LITER OF AQUEOUS SOLUTION, OF ABOUT 40 TO ABOUT 80 GRAMS OF POTASSIUM AUROCYANIDE, OF ABOUT 6 TO ABOUT 9 GRAMS OF POTASSIUM CYANIDE AND OF ABOUT 32 TO ABOUT 48 GRAMS OF POTASSIUM HYDROXIDE; AND PASSING AN ELECTRIC CURRENT THROUGH THE SOLUTION WITH THE SUBSTRATE TO BE PLATED BEING THE CATHODE AND THE ANODE BEING THE INSOLUBLE TYPE. 